I miss you - you say this to someone who is away from you - you miss them I miss you already - say you were farewelling someone at the airport,, and you know when they are gone you will miss them. Here you are saying I miss you already - you haven't even gone yet and I am already feeling sad because I know how much I am going to miss you. I missed you is what you would say when the person returns - it is past tense. I missed you (while you were gone), but you are back now, so I am no longer missing you. I have miss you - this isn't grammatical. It should be "I have missed you". Again it is what you would say when someone comes back and you are no longer missing them.